A fuel cell is an electrochemical device that produces electrical current from chemical reactions. The fundamental device includes an ion-conducting electrolyte between two electrodes, backed by fuel and oxidant flow distributors. A catalyst on one electrode promotes separation of ions and electrons at the oxidant side. Only the ions conduct through the electrolyte, and recombine with electrons at the fuel side. The electrons are conducted through an external circuit, thus supplying electrical power. Solid oxide fuel cells have ionic-conducting metal oxide membranes as their electrolyte layer. The oxygen molecules are split into their respective electrons and oxygen ions at the airside. The oxygen ions propagate through the electrolyte membrane and combine with their electrons and hydrogen molecules into water.
Fuel cell operation is increasingly efficient where the well-known electron conductivity of the electrolyte is brought to a minimum and the well-known ionic conductivity of the electrolyte is brought to a maximum. At the same time it is desirable to keep the fuel cell's driving temperature as low as possible such that the well-known thermodynamic efficiency and the well-known electric load responsiveness of the fuel cell remains at high levels.
As is well-known in the art, ohmic resistance and ionic conductivity of conventional electrolyte materials vary with electrolyte temperature. In that context, Table 1 lists exemplary values of ionic conductivity σ [S/cm], ohmic resistance of electrolyte film at 1 cm2 of area and at given film thickness R [Ω] for various yttria stabilized zirconium [YSZ] film, thickness of various YSZ film h [μm, nm] and temperatures t.
TABLE 1R [Ω]R [Ω]R [Ω]R [Ω]tforforforfor[° C.]σ [S/cm]h = 5 μmh = 500 nmh = 200 nmh = 20 nm5030.001                4221.58e−4    3.1           3603.00e−5   16.6   1.66       3228.00e−6   62.5   6.25   2.5   2505.11e−7   978   97.8  39.1  3.92004.54e−8  11010  1101  440  441004.88e−1110245900102459040983640983
The bold and italic entries of Table 1 indicate ohmic values relevant for efficient fuel cell operation. Hence, by reducing the electrolyte height, the driving temperature, the ohmic resistance and the ionic conductance are reduced as well.
At the time the present invention was made, extensive effort has been dedicated to reduce electrolyte thickness. In one well-known tape casting technique for example, slurry of submicrometer-sized powders is used to produce highly dense and mechanically strong electrolyte films with good electrical properties. Unfortunately, the electrolyte films are fabricated only with relatively large heights in the range of hundreds of micrometers. This reduces the ionic conductance across the electrolytes' heights to relatively low levels that may be only partially compensated by high working temperatures of 800–1000° C.
In another approach, electrolyte films are fabricated as anode-supported thin oxide films with electrolyte heights between 5–20 μm resulting in a working temperature range of 500–1000° C. Even though a significant reduction of the lower working temperature limit was accomplished, 500° C. require still significant constructive effort and limit the feasibility of such a fuel cell for practical applications.
At the time of the invention, high-tech processing such as well-known PVD, CVD, PLD and sol-gel deposition has been tested to further reduce electrolyte thickness and increase film density. Since the electrolyte operates also as a membrane that physically separates reactant fluids within the fuel cell, film density becomes more important with decreasing electrolyte thickness to provide gas impermeability at sufficiently high levels. Unfortunately, the high-tech deposition technologies have been developed mainly for highly flattened substrates, especially silicon wafers. To the contrary, a substrate qualifying for deposition of an electrolyte membrane must be highly gas permeable to provide for fluid conductance through the substrate and for a direct fluid contact with the electrolyte at the electrolyte side adjacent the substrate. A qualifying substrate needs to be highly porous, which results in a relatively rough, discontinuous and inhomogeneous surface on a scale similar to that of the electrolyte's film height.
A successful deposition of continuous submicron YSZ thin films on anodized nanoporous alumina has been demonstrated in the prior art by a sol-gel deposition technique using viscous alkoxide-derived solution applied on top of the porous substrate. However, sol-gel derived thin films exhibit a large shrinkage during heat-treatment and low density from inherent high organic content that may cause local defects. Consequently, continuous films that are substantially fluid impermeable may not be fabricated with sol-gel deposition techniques.
In summary, prior art oxide film deposition techniques are not suitable in combination with a qualifying substrate. Therefore, there exists a need for a fabrication technique for making an operational electrolyte membrane on top of a porous substrate. The present invention addresses this need.
In general, an electrolyte layer of a solid oxide fuel cell should have the following properties:                high ionic conductivity, which means also low ionic resistance;        low electron conductivity, preferably by reduction of thickness;        high density and impermeability to prevent electric potential drop between both sides of the electrolyte layer due to reactant fluid mixture;        sufficient mechanical strength at operational fluid pressures; and        good adhesion to electrode layers to reduce resistance between electrolyte and electrode for increased fuel cell efficiency and to prevent second phase formation.        
The present invention addresses these general needs.
In particular, an electrolyte layer is needed that may be fabricated in an inexpensive fashion with a configuration that provides for an efficient fuel cell operation at working temperatures of 500° C. and substantially less. The present invention addresses also these needs.